You all know of Sisyphus as the guy who had to roll a boulder uphill, only to have it roll back down again. He was doomed to do this pointless activity over and over, for all eternity. He gets a lot of sympathy from modern office workers.
What's less well known is that Sisyphus got what was coming to him. His famous fate was an addendum to what, to the Greeks, was a much more important story.
The original Sisyphus of legend was the first king of Corinth, and he was a nasty and sneaky individual. He used to murder people who passed through his land, just for the lulz.
Sisyphus hated his brother Salmoneus. Sisyphus was told by an oracle that if he had children by the daughter of Salmoneus, a woman named Tyro, then the children would grow to kill their grandfather. So Sisyphus seduced his niece (this while he was married, mind you). But Tyro learned of the oracle and killed her own children to prevent them killing her father.
While this was seriously antisocial, what eventually got Sisyphus into major trouble was when he divulged some of Zeus's more embarrassing indiscretions to the world at large.
The king of the gods was not amused. He sent Thanatos, the god of death, to cart Sisyphus off to Hades. (There are different versions of this story. In some, it's Hades himself who turns up to snaffle Sisyphus. Thanatos was a minor god who doesn't usually get much airplay.)
Thanatos duly arrived to collect his victim, bearing with him chains in which to wrap Sisyphus. Sisyphus expressed great interest in how the chains worked and asked for a demonstration. Thanatos obliged, using himself as the subject. Sisyphus instantly caught up the chained Thanatos and threw him in the palace cupboard. Sisyphus then carried on in the living world for some years. Meanwhile Thanatos was stuck in the cupboard, no doubt doing multiple facepalms.
But with Thanatos out of business no one ever died, which upset the balance of the world. Eventually Ares the god of war got sick of battles in which no one died, no matter how often they were skewered with spears. Ares went to free Thanatos, and Sisyphus was sent to Hades.
It doesn't end there. Sisyphus sweet-talked the goddess Persephone, queen of the dead, into letting him back up again. There are multiple versions of how he did this, but the usual is he ordered his wife not to give him a proper burial, then convinced Persephone he had to return to the world to arrange his own funeral.
So Sisyphus returned to the world and carried on with riotous living. Leaving Persephone to wait for Sisyphus to return to Hades, and she waited a long time, no doubt doing multiple facepalms.
Zeus eventually realized that if you want something done right, then you have to do it yourself. He carried Sisyphus off to Hades and set up the boulder scheme.
And that's why Sisyphus is still down there, pushing that boulder uphill.